Particle beam therapy is a therapeutic method that has been developed primarily for the treatment of certain cancers, in preference to conventional x-ray radiation therapy. The technique uses an energetic beam of ions to target the tumor with a higher degree of precision, making it the treatment of choice for some cancers. The specific property of the ion beam is its ability to deliver dose deep into the target with minimal dose to the tissues in front or in back of the target position.
The effectiveness of the method is dependent on the ability of the delivery system to accurately deliver a specified, three-dimensional distribution of dose. This task can be divided into two parts. The first part is to control the depth of the beam into the target. This is done through control of the beam energy. The second part is the need to control the beam in the lateral dimension, perpendicular to the beam direction.
In the method that has dominated the field, known as Double Scattering, the beam is formed into a wide beam with dimensions larger than the target tumor. Collimators near the patient are used to define the lateral shape of the beam. Typically, the beam energy is periodically modulated in a number of discrete steps and over a time scale of 100 milliseconds though an energy degrader mechanism. In this way, the entire volume of the tumor is irradiated “at once”. When properly executed, a double-scattering treatment will proceed until a predetermined total dose has been delivered
In the newer method known as Pencil-Beam Scanning, a much narrower beam is used to “draw” the three-dimensional dose distribution. Lateral positioning of the beam is done through the use of a magnetic deflection system. Typically, the treatment is divided into discrete energy steps, in which a two-dimensional distribution is delivered for each energy in turn, building up the full three-dimensional distribution as the treatment progresses.
PBS is becoming the mode of preference due to the elimination of the need for physical collimators and other hardware, and its ability to create arbitrary three-dimensional distributions, a capability that does not exist for double scattering.
PBS is typically operated in “spot-scanning” mode in which the beam is directed to the target lateral position with the beam disabled. The beam is then enabled and a predetermined target dose is delivered. The beam is then disabled, which results in an actual dose that may have a deviation from the specified target. An appropriate characterization of, and reaction to such errors is a key aspect of this patent.
Any treatment method requires a control system to execute the desired treatment. An important aspect of such a control system is the ability to monitor the progress of the treatment to ensure accuracy of the final dose distribution, and to react to process deviations that could result in a potentially dangerous dose error.
The monitoring function requires the ability to quantify the beam delivered to the patient during treatment. In the case of double scattering, this takes the form of an inline detector to verify the uniformity of the scattered beam at the collimator position, in addition to a total dose detector.
In the case of pencil-beam scanning, the similar monitoring hardware is typically used, with minimal changes in the monitoring software. As a result, the specific process parameters that arise through the use of PBS are not properly accounted for. As a result, existing PBS systems exhibit a number of problematic characteristics, chief among them a high rate of process interruptions.
Elimination of the existing problems with the PBS monitoring process requires the design of innovative new hardware, and the application of calculation-intensive real-time analysis algorithms. The general approach is to measure both the beam trajectory and the beam shape at each moment in the scanning process, and to incorporate this information into an analysis that properly predicts the dose distribution actually delivered to the patient as the process is occurring.
The primary measurement needed in the monitoring and control of a therapeutic ion beam is that of the beam position and intensity profile at various points in space. One generally accepted method for measuring charged particle (e.g., protons or other ions) beam current is through the use of a transmission ion chamber. In its simplest implementation, this detector consists of two planar electrodes arranged in a parallel configuration spanning a gas-filled layer. A bias voltage is applied between the two electrodes to establish an electric field in the air gap. Current or charge-integrating electronics is attached to one of the electrodes. An example of such a transmission ion chamber detector is illustrated in FIG. 1. Because scanned-beam applications span a large area at the patient position, any ion chambers placed after the scanning magnets must generally be large in area.
As a beam of ionizing radiation (such as charged particles, including protons) passes through the detector in a direction nominally perpendicular to the electrode plane, some fraction of the incident beam energy is lost to the fill gas. The energy lost is a function of incident radiation type, radiation energy, the thickness of the gas gap, and the density of the gas. Given these parameters, the energy loss can be accurately calculated.
One mechanism for the energy loss in the gas is the through ionization, the creation of electron-ion pairs. It has been determined experimentally that there is a fixed relationship between the energy loss and the number of ion-pairs created. This is typically referred to as “W” and has a value of approximately 32 eV/Ion pair. To good accuracy, the rate of creation of electron-ion pairs is proportional to the intensity of the incident irradiation, and the constant of proportionality can be theoretically determined. It is this property, combined with the ease of calculation of energy loss, that has led to the use of the ion chamber in applications requiring accurate, quantitative, and easily calibrated measurement of radiation intensity.
In isolation, these electron-ion pairs would ordinarily recombine in a short time, typically in less than 1 msec. However, the imposed electric field causes the charges to separate and move towards the electrodes, with the electrons moving toward the positive potential, and the positive ions moving toward the negative electrode. An electric field of about 1000 V/cm is typically adequate to collect close to 100% of the generated charge, with a small percentage being lost to recombination.
The ion chamber has a gain, a fixed ratio relating the intensity of the incident radiation to the collection ion current. The gain is dependent on the gas density, but has very little dependence on the bias voltage (as long as it is high enough) or the gas species. The gain is energy dependent, but is fixed for any specific beam energy.
There are several variants from the planar detector described above. In general, the planar electrode can be divided into an arbitrary arrangement of smaller electrodes. Each of these new component electrodes will collect charge from the 3-dimensional gas volume defined by the electrode shape projected along the electric field lines. The total collected charge is unchanged from the original configuration. Such subdivisions of the electrode allow the determination of the spatial distribution of the incident radiation beam.
One useful configuration is a strip detector, which is illustrated in FIG. 2. Here, the strip detector 200 includes an electrode plane 210 divided into long, narrow strips 215. The geometry of the strip detector is such that each strip 215 collects charge from a particular lateral position of the beam, but independent of the position in a perpendicular axis 220. This provides the distribution of beam intensity projected onto the perpendicular axis 220. Such detectors can be used in orthogonal pairs, in which case the projection of the beam intensity is determined for two orthogonal axes. This measurement determines the centroid position of the beam in the X, Y plane, since this calculation is dependent only on the orthogonal projection data, and does not require the full 2-dimensional distribution of the beam intensity.
The construction and readout of the strip ion chamber 200 is simplified by the fact that the electrode signal can be extracted at the ends 240 of the electrode strips 215, where an attachment mechanism 250 is outside of an active area 260 of the detector 200. The relatively small number of sensing electrode strips 215 allows the sensing electronics to remain simple.
In cases in which the full two-dimensional distribution of the charged particle beam cross-section is needed, the electrode plane can be divided into an array of square or rectangular sub-electrodes or “pixels.” Here, the problem of extracting the signals is more complex. It is necessary to bring the signals from the individual pixels to a point of external connection through conductive traces.
One solution is to create traces on the surface that contains the pixels themselves. A shortcoming of this approach is that these conductive traces will themselves become collecting electrodes, distorting the resulting data. However, this approach simplifies the construction, since the traces can be created as part of the processing step that is used to create the pixels. This approach adds no additional material to the detector, and so can provide the thinnest construction.
An alternative solution is to dispose the conductive traces on the back side of the sensing electrode, where there is no electric field, and therefore no charge collection on the traces. This requires the use of through-plane electrical “vias” to connect the rear traces to the individual pixels. FIG. 3 illustrates a front side 310 and a back side 320 of a pixelated ion chamber detector 300. This method is less attractive, requiring more complex manufacturing processes.
For applications requiring small beam spots at the patient position, an important specification of the ion chamber is its effect on the beam spot size through the mechanism of scattering. Scattering increases the beam's emittance, which is a measure of the optical quality of the beam, and relates directly to the ability to focus the beam to a small spot. The location of the scattering object is also important as the further the beam is from the target when the scattering occurs, in general the more pronounced its effect is at the target location.
For the purpose of measuring the beam cross-sectional intensity distribution, it is preferable to measure the beam shape close to the patient, since this is the measurement most representative of the beam striking the patient. This is impractical because of the resulting large number of pixels required to span the large scan area (typically 30×40 cm) with adequate resolution. It is advantageous to monitor the beam as close as possible, but preceding the scan magnet because the beam is small and stationary, allowing the use of a small detector, a tractable number of pixels (128-1024), and simple electronics. However, this location is far enough from isocenter that scattering is a severe problem, and sufficiently thin ion chambers have not been available from this application. The initial development of particle therapy used mainly passive lateral scattering followed by collimation to conform the beam of particles to the dimensions of the target (e.g., a tumor). In this respect, it is similar to X-ray radiotherapy, in which a broad fan of X-rays is collimated to conform to the target dimensions.
Since depth of treatment is a function of particle energy, any treatment method must modulate the beam energy. In the commonly used “double scattering” technique, the beam energy is modulated at high speed, and a broad beam is delivered to the patient position, spanning the entire tumor volume at the same time. In the newer, scanned beam method, the area of the tumor is drawn with a narrow beam typically at a single energy, and this process is repeated for each of a set of energies, thus building up the required depth profile.
In general, scattered particle therapy treatment systems rely on arrangements calculated and made before the treatment starts to control the lateral distribution of dose, for example the use of custom-machined collimating apertures. Thus, while the treatment is in progress, the supervising therapist need only monitor the accumulation of overall dose (e.g., by monitoring a dose counter) to ensure that it reaches the specified total, at an acceptable rate, without significant under- or overdose. In this respect, the methods of dose delivery monitoring and therapist oversight of the process are the same as those used for X-ray radiotherapy, to the extent that the applicable standards are identical or closely related.
A more recent means of delivering conformal dose with particle beams is the use of active beam deflection, or scanning, often called pencil beam scanning (PBS). A small beam spot with adjustable intensity is moved over the target area by deflecting a mono-energetic beam using fast electromagnets, according to a pre-calculated trajectory. The process is repeated for each of a set of energies. This method provides a finer control of where dose is delivered, and allows essentially arbitrary lateral distribution profiles, unconstrained by the mechanical limitations imposed by collimating apertures. The lack of such apertures also reduces the generation of unwanted neutrons close to the patient, which produce untargeted radiation dose. Pencil beam scanning is an essential component of most new particle beam therapy installations, and seems destined to become the primary dose delivery means.
PBS introduces a new complication to the monitoring, interlocking and oversight of the dose delivery. Since the PBS beam covers only a small fraction of the tumor at any given time, in order to irradiate the width of the tumor, the PBS beam must be scanned laterally across a plane that is perpendicular to the PBS beam.
Although PBS beam scanning introduces new error sources (e.g., the position of the PBS beam spot on the x-y treatment plane, present system control has not advanced significantly to deal with this new delivery mode. PBS systems still rely on the total dose as an important parameter allowing direct monitoring by the operator. In the commonly used “spot-scanning” mode of PBS, the dose is delivered to discrete locations in sequence. At each such spot, the dose is monitored, with the goal of delivering a particular dose. The error in the delivered dose is monitored, and if it exceeds a preset value, an error condition is generated, resulting in a pause or stop to the process, as well as operator intervention. Because this method looks at one spot at a time, it is not able to consider spatially overlapping, compensating errors. No method now in use monitors the overall delivered distribution for excessive deviation during treatment.
A weakness in existing PBS delivery systems is that the actual dose distribution, such as across the x-y treatment plane for a given energy level, is not determined during treatment. As a result, it is necessary to apply very stringent limits on delivered dose on individual spots. The single-spot monitoring method introduces an excessive number of error conditions. The tendency for overlapping errors to average out is not considered. Such consideration requires the sort of 2d analysis that is the basis of the technique.
A common technique for PBS monitoring is to execute the full treatment plan without the patient present, applying the dose to a volume of water to simulate a therapeutic scan of a patient's tumor, and then comparing the resulting dose to the target distribution. While such a technique can be useful to verify the overall correctness of the treatment plan, it does nothing to account for real-time variations during therapeutic treatment of a patient.
Accordingly, there is a need for PBS systems that do not suffer from some or all of the above problems.